Conventionally, electrical signals have been widely used in data transmission over short distances by computers and the like. In recent years, as the clock frequency of computer central processing units (CPUs) has increased, electrical signals have also come to be used in high frequency bands.
However, delays and noise from electrical wiring and the like have started to become a problem in such data transmission by electrical signals in high frequency bands. As a result, data transmission using light has begun to replace electrical signals. Optical fiber is used as the transmission path in data transmission using light, which is generally divided into multi-mode data transmission over short distances of a few hundred meters or less and single mode data transmission over longer distances of a few hundred kilometers or more. Different optical fibers are also used in multi-mode and single mode transmission: while in multi-mode transmission the diameter of the high refractive index part (hereunder, “the core”) ranges from about 50 to 60 μm, in single mode transmission the diameter of the high refractive index part is about 7 μm, or narrower than that of the multi-mode core. A smaller core is used in single mode transmission in order to inhibit the occurrence of modes with differing optical propagation path lengths within the core, so that the waveform of the optical signal will not break down even with long-distance transmission. By contrast, because the transmission distance is shorter in multi-mode transmission the existence of modes with differing propagation path lengths is not so much of a problem.
In such optical transmission, optical transmission modules are used to convert electrical signals to optical signals and optical signals to electrical signals. Optical transmission modules are equipped with light-emitting elements, light-receiving elements or both. Semiconductor laser elements and vertical cavity surface emitting laser (VCSEL) elements in particular are widely used as the aforementioned multi-mode light-emitting elements.
An ordinary commercial optical transmission module is VCSEL module, in which the laser light is focussed by a lens which is an integral part of the sealed window of the vertical cavity surface emitting laser (VCSEL), and optically coupled to optical fiber on the outside.
There has also been an example in which the laser element, drive circuit, photodetector, current-voltage conversion circuit, integrally-formed lens for sealing and the like were integrated into one package as an optical transmission module. Patent Reference 1: Japanese Patent Application Laid-open No. H10-126002
Moreover, there has also been an example of a semiconductor laser module for optical disk recording and reproduction in which the laser element, light-emitting window, case, collimator lens and light-receiving element near the light-emitting window were integrated into a single module. Patent Reference 2: Japanese Patent Application Laid-open No. 10-303513
Moreover, there has also been an example of an optical element module in which a structure (face plate structure) designed to control convergence by means of bundled fibers finer than the optical fibers used in transmission was placed between the optical element and the optical fiber for transmission. In this example, there is no need for the optical element to be aligned with the faceplate or the faceplate with the optical fiber for transmission. Patent Reference 3: Japanese Patent Application Laid-open No. 10-231040
However, because in ordinary commercial optical transmission modules, the light emitted by the VCSEL laser is focussed by a lens, there is a need for alignment in order to achieve optical coupling with an external optical waveguide means, and costs are increased because a structure adapted to the optical module purchased must be separately designed and manufactured. Moreover, commercial modules only emit light and do not have a means for receiving light.
Moreover, in optical transmission modules having a structure in which a semiconductor laser element, a light-receiving element and the like are assembled in a package and sealed with a light-transmitting window which is integrated with a lens, the light-transmitting window cannot be aligned and there is no means of correction if the lens is out of alignment with the laser element.
In addition, a structure has been disclosed for a semiconductor laser for optical disk recording and reproduction in which a light detector or wave front conversion element is formed in combination with the light-transmitting window, which has a seal structure. Moreover, a structure has also been disclosed in which laser light is converted to parallel rays with a collimator lens, the collimator lens is also provided with a reflective surface and the reflected light is received, but both of these only monitor the strength of light emitted by the light source itself.
Moreover, in an optical element module in which a structure (face plate) which controls divergence by means of bundles of fibers finer than optical fibers is placed between the optical element and the optical fiber, the face plate could also be used as the sealing window of the optical element, but there is a problem of increased optical loss because of the existence of dead space between the fine fibers of an optical path composed of bundled fibers. Because in optical wiring the S/N ratio is reduced to the extent that the power of the light source is attenuated, the optical coupling efficiency between the optical element and optical fiber should be as high as possible. Moreover, even when a face plate structure is used it is necessary to align the optical element with the optical fiber for transmission.
Therefore, it is an object of the present invention to provide an optical transmission module capable of providing highly efficient optical coupling between an optical element and an external optical transmission means, wherein the alignment therefor is easy and the external optical transmission means can be easily attached and detached, together with a manufacturing method therefor.